IQ isn't used for anything meaningful in terms of ability or qualifications, especially since it's age-adjusted and historically has been biased against minorities/foreigners. A year of development however, is a graspable (if shocking) measure of lost opportunity/capability.
I have no idea at all. I'm not trying to be snarky, I completely failed to grasp what's being lost until I went and looked at the paper.
A year of education is certainly quantifiable when we're talking about learned facts and skills, or even about economic success, but I don't understand it as a measure of cognitive impairment. The studied harms should still be relevant to a farmer who's never spent a single day in a classroom, and the analysis in the study focuses on older adults losing fundamental capacities that are important to decision-making and daily functioning. Using a metric that's based on education doesn't clearly convey losses like that, especially when we're talking about not missed advancement in children but decline in older adults.
The actual result obtained in this study was performance on a set of scores on verbal and mathematical tests relative to a population average. The result in the lead study was a set of verbal and mathematical tests relative to a population average. Expressing one as percentiles and the other as IQ doesn't change the fact that those things are far more similar to each other than either is to keeping children in school for an extra year.
(On a final note, I don't want to open the IQ debate, but I do think the usual problems are uncommonly irrelevant here. I specifically pulled a lead study which was conducted in China, used a Chinese baseline, and compared among same-age children, which avoids the biases mentioned. IQ may be a terrible general comparison, but if we don't need to compare between samples in different studies then it's really just a percentile.)
Is it? What people remember of one year of education is probably very little to begin with and it only gets less over time; it doesn't seem unbelievable that pollution could cause those memories to decay faster.
That said, modern IQ doesn't use that definition, and all of the material you can easily find on the matter pretends that it does. Instead, IQ is a normalized bell curve, every 15 points is another standard deviation on the bell curve. Given that definition, a year's worth of education is probably guesstimated at roughly one standard deviation of IQ. This comes with the massive caveat that IQ and educational achievement can't be compared in that manner (since one is an absolute scale and the other is definitionally a statistical artifact).